had a frein back in the mid 70's with a warmed over 383 in a 63 Plymouth. Always ran hot. Hid tried all kinds of things to no avail. When he swapped a 440 in he found the root cause. The water pump impeller was laying in the housing
We weere pretty young and didn't think to check for water movement at 2-3k RPM

A few years ago I had a street rod come in the shop with a new engine , trans, radiator etc. and a heating problem. had been to a couple of different places with no success. It had a 90° elbow on the top tank so you could not really see the coolant. A borescope showed little movement of the coolant at 2500 RPM. After checking pulley ratios, the thermostat etc, we found a red shop towel inside the lower radiator hose All was well after removing it